Resin for optical-semiconductor-element encapsulation containing polyimide and optical semiconductor device obtained with the same

ABSTRACT

The present invention relates to a resin for optical-semiconductor-element encapsulation containing a polyimide which is produced by imidizing a polyimide precursor obtained by subjecting 5-norbornene-2,3-dicarboxylic anhydride or maleic anhydride, an aliphatic tetracarboxylic dianhydride, and an aliphatic diamine compound to a condensation polymerization reaction. The resin of the invention has excellent heat resistance and excellent light-transmitting properties. In addition, the present invention also relates to an optical semiconductor device containing an optical semiconductor element encapsulated with the resin.

FIELD OF THE INVENTION

The present invention relates to a resin foroptical-semiconductor-element encapsulation which contains a polyimide;and to an optical semiconductor device containing an opticalsemiconductor element encapsulated with the resin.

BACKGROUND OF THE INVENTION

With the recent trend toward improvements in luminous efficiency andluminous flux and increase in current density in light-emitting diodes(LEDs), the deterioration of LED encapsulation resins has become aproblem because it is causative of LED life shortening. In general,epoxy resins are frequently used as LED encapsulation resins. However,it is known that the enhanced heat generation due to the increase ininjection current density further accelerates the deterioration of theepoxy resins.

Specifically, examples of the resins for use in the encapsulation ofoptical semiconductor elements include an alicyclic epoxy resin (see,JP-A-7-309927) and a polyimide-containing resin obtained by reacting anaromatic acid dianhydride with an aromatic diamine compound (see,JP-A-63-7657).

However, alicyclic epoxy resins still have insufficient heat resistance,while polyimide-containing resins obtained by reacting an aromatic aciddianhydride with an aromatic diamine compound are not sufficient in heatresistance and light-transmitting properties.

SUMMARY OF THE INVENTION

An object of the invention is to provide a polyimide-containing resinfor optical-semiconductor-element encapsulation which has excellent heatresistance and excellent light-transmitting properties. Another objectof the invention is to provide an optical semiconductor devicecontaining an optical semiconductor element encapsulated with the resin.

Namely, the invention relates to the following items 1 to 7.

1. A resin for optical-semiconductor-element encapsulation comprising apolyimide which is produced by imidizing a polyimide precursor obtainedby subjecting 5-norbornene-2,3-dicarboxylic anhydride or maleicanhydride, an aliphatic tetracarboxylic dianhydride, and an aliphaticdiamine compound to a condensation polymerization reaction.

2. The resin for optical-semiconductor-element encapsulation accordingto item 1, wherein the aliphatic tetracarboxylic dianhydride is1,2,3,4-butanetetracarboxylic dianhydride or1,2,3,4-cyclopentanetetracarboxylic dianhydride.

3. The resin for optical-semiconductor-element encapsulation accordingto item 1, wherein the aliphatic diamine compound is at least one memberselected from the group consisting of the compounds represented by thefollowing formulae (I) to (V):

wherein n is an integer of 4 to 18, and

4. The resin for optical-semiconductor-element encapsulation accordingto item 1, wherein the polyimide precursor has a number-averagemolecular weight of 3,000 to 50,000.

5. The resin for optical-semiconductor-element encapsulation accordingto item 1, wherein the polyimide precursor has a degree ofpolymerization of 5 to 1,000.

6. The resin for optical-semiconductor-element encapsulation accordingto item 1, which has a light transmittance, as measured at a wavelengthof 400 nm, of 99% or higher.

7. An optical semiconductor device comprising an optical semiconductorelement encapsulated with the resin according to item 1.

The resin for optical-semiconductor-element encapsulation according tothe invention has excellent heat resistance and excellentlight-transmitting properties. Consequently, the optical semiconductordevice containing an optical semiconductor element encapsulated withthis resin has excellent heat resistance and excellentlight-transmitting properties.

DETAILED DESCRIPTION OF THE INVENTION

The resin for optical-semiconductor-element encapsulation of theinvention contains a polyimide which is obtained by imidizing apolyimide precursor obtained by subjecting 5-norbornene-2,3-dicarboxylicanhydride or maleic anhydride, an aliphatic tetracarboxylic dianhydride,and an aliphatic diamine compound to a condensation polymerizationreaction.

Examples of the aliphatic tetracarboxylic dianhydride include the samealiphatic tetracarboxylic dianhydrides as those which have been used forproducing polyimides and polyimide precursors. However,1,2,3,4-butanetetracarboxylic dianhydride and1,2,3,4-cyclopentanetetracarboxylic dianhydride are preferred from thestandpoints of the heat resistance and transparency of the product,availability, and handleability. Such dianhydrides may be used alone orin combination of two or more thereof.

Examples of the aliphatic diamine compound include the same aliphaticdiamine compounds as those which have been used for producing polyimidesand polyimide precursors. However, from the standpoints of the heatresistance and transparency of the product, availability, andhandleability, it is preferred that the diamine compound be at least onemember selected from the group consisting of the compounds representedby the following formulae (I) to (V).

(In the formula (IV), n is preferably an integer of 4 to 18, and is morepreferably an integer of 4 to 8.)

These diamine compounds may be used alone or in combination of two ormore thereof.

The resin for encapsulation according to the invention may be oneproduced from the starting materials which further include a diaminecompound other than the aliphatic diamine compounds represented by theformulae (I) to (V), so long as use of such a diamine compound does notimpair the effects of the invention. However, from the standpoint ofobtaining excellent light-transmitting properties and excellent heatresistance, which is an object of the invention, the use amount of thealiphatic diamine compound represented by the formulae (I) to (V) ispreferably 80 to 100% by weight, more preferably 90 to 100% by weight,even more preferably 95 to 100% by weight, based on the mixture of thediamine compound and aliphatic diamine compound to be subjected to thereaction.

In the condensation polymerization reaction in the invention, it ispreferred that the starting materials be mixed so that the molar ratioamong the compounds satisfies the following relationship:

{(number of moles of aliphatic tetracarboxylic dianhydride)+[1/2×(numberof moles of 5-norbornene-2,3-dicarboxylic anhydride or maleicanhydride)]}:(number of moles of aliphatic diamine compound)issubstantially 1:1.

Examples of organic solvents usable in the condensation polymerizationreaction include N,N-dimethylformamide, N,N-dimethylacetamide,N-methyl-2-pyrrolidone, N-methylcaprolactam, dimethyl sulfoxide,pyridine, tetrahydrofuran, cyclohexanone, and 1,4-dioxane. Thesesolvents may be used alone or in combination of two or more thereof.

The polyimide precursor obtained by subjecting those compounds to acondensation polymerization reaction preferably has a number-averagemolecular weight of 3,000 to 50,000. In this specification, thenumber-average molecular weights of polyimide precursors are onesdetermined by nuclear magnetic resonance spectroscopy (NMR).

Examples of the imidization reaction include thermal imidizationreaction and imidization reaction caused by a chemical reaction, and inthe invention, it is preferred to conduct thermal imidization reaction.The thermal imidization reaction is conducted in the temperature rangeof preferably 50 to 400° C., more preferably 100 to 250° C. Thisreaction may be performed continuously, and may be performed undervacuum or in an inert gas atmosphere. In the invention, the reaction maybe conducted, for example, at 100° C. for 1 hour, subsequently at 150°C. for 1 hour, and then at 200° C. for 1 hour, from the standpoint ofmaintaining colorlessness and transparency, thereby obtaining apolyimide-containing resin for optical-semiconductor-elementencapsulation.

In the case where the polyimide precursor is imidized in a sheet form,the following method can, for example, be used. A solution of thepolyimide precursor is diluted with an organic solvent such asN,N-dimethylacetamide or N-methyl-2-pyrrolidone according to thenecessity. The solution is then applied in an appropriate thickness to arelease sheet (e.g. polyester film), glass substrate, or the like whichhas a silicone-treated surface, by a technique such as casting, spincoating, or roll coating. The polyimide precursor solution applied issubjected to the thermal imidization reaction, whereby a sheet-formresin can be obtained.

The polyimide precursor or the polyimide obtained by imidizing theprecursor, when subjected to structural analysis by NMR or IRspectroscopy, show the following 2 0 characteristic peaks.

Polyimide precursor: (′H NMR (DMSO-d₆ solution): δ 12.3, 8.3-7.5,6.2-5.9, 3.7-2.6, 2.0-0.7)

Polyimide: (IR: 1702-1678, 1407-1381 cm⁻¹)

Furthermore, the degree of polymerization n of the polyimide precursor,as determined from integral ratio between the signal attributable tomain-chain NH and the signal attributable to a terminal olefin, ispreferably 5 to 1,000, more preferably 9 to 200.

In the case where the polyimide-containing resin foroptical-semiconductor-element encapsulation has a sheet form having athickness of, e.g., 10 to 200 μm, the light transmittance thereof, asmeasured at a wavelength of 400 nm, is preferably 90% or higher, morepreferably 95% or higher, even more preferably 99% or higher, from thestandpoint of the durability of the optical semiconductor device. Inthis specification, light transmittance is measured by the methoddescribed in the Examples which will be given later.

The polyimide-containing resin thus obtained has excellentlight-transmitting properties and excellent heat resistance. It is hencesuitable for use as a resin for optical-semiconductor-elementencapsulation, which is used in, for example, an optical semiconductordevice including a blue or white LED element mounted therein (e.g., abacklight for liquid-crystal screens, traffic signal, large outdoordisplay, or advertising signboard). Consequently, the invention furtherprovides an optical semiconductor device containing an opticalsemiconductor element encapsulated with the resin foroptical-semiconductor-element encapsulation.

Since the above-mentioned resin has a double bond at each molecular end,a silicone resin, epoxy resin, acrylic resin, or the like can be bondedto the molecular end thereof according to the necessity to therebyregulate flexibility, adhesiveness, moisture resistance, etc.

In general, a resin having no acid anhydride in the molecule thereof hasan acid anhydride group or amino group at each molecular end. Because ofthis, when the silicone resin, epoxy resin, acrylic resin, or the likeas mentioned above is to be bonded to such a resin, it is difficult todesign a reaction. In contrast, since each molecular end of the resinaccording to the invention always has a double bond, reaction design iseasy.

The optical semiconductor device of the invention can be produced byencapsulating an optical semiconductor element such as an LED elementwith the resin for optical-semiconductor-element encapsulation of theinvention, i.e., with the polyimide precursor which has not beenimidized or with the polyimide obtained by imidizing the polyimideprecursor. Specifically, in the case where the polyimide precursor isused, the polyimide precursor is applied as it is to a substrate havingan LED element mounted thereon in an appropriate thickness by atechnique such as casting, spin coating, or roll coating, and theprecursor applied is then heated and dried to thereby produce an opticalsemiconductor device. In the case of using the polyimide, an opticalsemiconductor device can be produced, for example, by superposing apolyimide-containing sheet formed by a technique such as casting, spincoating, or roll coating so as to have an appropriate thickness on asubstrate having an LED element mounted thereon and then laminating thesheet to the substrate with a laminator or the like.

EXAMPLES Example 1

A dimethylacetamide solution (3.7 mL) of 1.56 g (10.1 mmol) of2,5(2,6)-bis(aminomethyl)bicyclo[2.2.1]heptane (NBDA: isomer mixture)was added dropwise to a dimethylacetamide solution (13 mL) of 1.78 g(8.99 mmol) of 1,2,3,4-butanetetracarboxylic dianhydride (BDA) and 0.328g (2.00 mmol) of 5-norbornene-2,3-dicarboxylic anhydride (NBA) at roomtemperature over 30 minutes. The resultant mixture was stirred at 60° C.for 24 hours to cause it to undergo a condensation polymerizationreaction and thereby obtain a solution of a polyimide precursorrepresented by the following structural formula. Part of the solutionwas poured into acetone with stirring. As a result, a white solidprecipitated. This solid was taken out by filtration, vacuum-dried, andthen analyzed for structure by 'H NMR spectroscopy and IR spectroscopy.

The degree of polymerization n thereof, as determined from integralratio between the signal attributable to main-chain NH and the signalattributable to a terminal olefin, was found to be 14 (number-averagemolecular weight, 5,400).

A substrate having a blue light-emitting diode was prepared. Thepolyimide precursor solution was applied by spin coating to thesubstrate surface including the blue light-emitting diode. The solutionapplied was heated at 100° C. for 1 hour, subsequently at 150° C. for 1hour, and then at 200° C. for 1 hour to imidize the precursor andencapsulate the blue light-emitting diode. Thus, a blue light-emittingdiode device was obtained.

The polyimide obtained by imidizing the polyimide precursor had thestructure represented by the following structural formula.

Example 2

A dimethylacetamide solution (10 mL) of 2.11 g (10.1 mmol) of4,4′-methylenebis(cyclohexylamine) (MBCA) was added dropwise to adimethylacetamide solution (10 mL) of 1.79 g (9.04 mmol) of1,2,3,4-butanetetracarboxylic dianhydride (BDA) and 0.327 g (1.99 mmol)of 5-norbornene-2,3-dicarboxylic anhydride (NBA) at room temperatureover 30 minutes. The resultant mixture was stirred at 60° C. for 24hours to cause it to undergo a condensation polymerization reaction andthereby obtain a solution of a polyimide precursor represented by thefollowing structural formula. Part of the solution was poured intoacetone with stirring. As a result, a white solid precipitated. Thissolid was taken out by filtration, vacuum-dried, and then analyzed forstructure by ¹H NMR spectroscopy and IR spectroscopy.

The degree of polymerization n thereof, as determined from integralratio between the signal attributable to main-chain NH and the signalattributable to a terminal olefin, was found to be 14 (number-averagemolecular weight, 6,200).

Using the polyimide precursor solution, a blue light-emitting diodedevice was obtained in the same manner as in Example 1. The polyimideobtained by imidizing the polyimide precursor had the structurerepresented by the following structural formula.

Example 3

A dimethylacetamide solution (4.6 mL) of 1.55 g (10.1 mmol) of2,5(2,6)-bis(aminomethyl)bicyclo[2.2.1]heptane (NBDA: isomer mixture)was added dropwise to a dimethylacetamide solution (8.0 mL) of 1.89 g(9.00 mmol) of 1,2,3,4-cyclopentanetetracarboxylic dianhydride (CPDA)and 0.328 g (2.00 mmol) of 5-norbornene-2,3-dicarboxylic anhydride (NBA)at room temperature over 30 minutes.

The resultant mixture was stirred at 60° C. for 24 hours to cause it toundergo a condensation polymerization reaction and thereby obtain asolution of a polyimide precursor represented by the followingstructural formula. Part of the solution was poured into acetone withstirring. As a result, a white solid precipitated. This solid was takenout by filtration, vacuum-dried, and then analyzed for structure by ¹HNMR spectroscopy and IR spectroscopy.

The degree of polymerization n thereof, as determined from integralratio between the signal attributable to main-chain NH and the signalattributable to a terminal olefin, was found to be 9 (number-averagemolecular weight, 3,700).

Using the polyimide precursor solution, a blue light-emitting diodedevice was obtained in the same manner as in Example 1. The polyimideobtained by imidizing the polyimide precursor had the structurerepresented by the following structural formula.

Example 4

A dimethylacetamide solution (3.3 mL) of 1.54 g (10.0 mmol) of2,5(2,6)-bis(aminomethyl)bicyclo[2.2.1]heptane (NBDA: isomer mixture)was added dropwise to a dimethylacetamide solution (13 mL) of 1.78 g(8.99 mmol) of 1,2,3,4-butanetetracarboxylic dianhydride (BDA) and 0.196g (2.00 mmol) of maleic anhydride at room temperature over 30 minutes.The resultant mixture was stirred at 60° C. for 24 hours to cause it toundergo a condensation polymerization reaction and thereby obtain asolution of a polyimide precursor represented by the followingstructural formula. Part of the solution was poured into acetone withstirring. As a result, a white solid precipitated. This solid was takenout by filtration, vacuum-dried, and then analyzed for structure by ¹HNMR spectroscopy and IR spectroscopy.

The degree of polymerization n thereof, as determined from integralratio between the signal attributable to main-chain NH and the signalattributable to a terminal olefin, was found to be 14 (number-averagemolecular weight, 5,300).

Using the polyimide precursor solution, a blue light-emitting diodedevice was obtained in the same manner as in Example 1. The polyimideobtained by imidizing the polyimide precursor had the structurerepresented by the following structural formula.

Example 5

A blue light-emitting diode device was obtained in the same manner as inExample 1, except that the amounts of the 1,2,3,4-butanetetracarboxylicdianhydride (BDA) and 5-norbornene-2,3-dicarboxylic anhydride (NBA) werechanged to 1.91 g (9.65 mmol) and 0.117 g (0.713 mmol), respectively, toobtain a solution of a polyimide precursor having a degree ofpolymerization n of 39 (number-average molecular weight, 14,200).

Example 6

A blue light-emitting diode device was obtained in the same manner as inExample 1, except that the amounts of the 1,2,3,4-butanetetracarboxylicdianhydride (BDA) and 5-norbornene-2,3-dicarboxylic anhydride (NBA) werechanged to 1.96 g (9.90 mmol) and 0.040 g (0.244 mmol), respectively, toobtain a solution of a polyimide precursor having a degree ofpolymerization n of 123 (number-average molecular weight, 43,800).

Example 7

A dimethylacetamide solution (3.7 mL) of 1.70 g (10.0 mmol) ofisophoronediamine (IPDA) was added dropwise to a dimethylacetamidesolution (13 mL) of 1.78 g (8.99 mmol) of 1,2,3,4-butanetetracarboxylicdianhydride (BDA) and 0.328 g (2.00 mmol) of5-norbornene-2,3-dicarboxylic anhydride (NBA) at room temperature over30 minutes. The resultant mixture was stirred at 60° C. for 24 hours tocause it to undergo a condensation polymerization reaction and therebyobtain a solution of a polyimide precursor represented by the followingstructural formula. Part of the solution was poured into acetone withstirring. As a result, a white solid precipitated. This solid was takenout by filtration, vacuum-dried, and then analyzed for structure by ¹HNMR spectroscopy and IR spectroscopy.

The degree of polymerization n thereof, as determined from integralratio between the signal attributable to main-chain NH and the signalattributable to a terminal olefin, was found to be 15 (number-averagemolecular weight, 5,600).

Using the polyimide precursor solution, a blue light-emitting diodedevice was obtained in the same manner as in Example 1. The polyimideobtained by imidizing the polyimide precursor had the structurerepresented by the following structural formula.

Example 8

A dimethylacetamide solution (3.7 mL) of 1.16 g (10.0 mmol) ofhexamethylenediamine (HMDA) was added dropwise to a dimethylacetamidesolution (13 mL) of 1.78 g (8.99 mmol) of 1,2,3,4-butanetetracarboxylicdianhydride (BDA) and 0.328 g (2.00 mmol) of5-norbornene-2,3-dicarboxylic anhydride (NBA) at room temperature over30 minutes. The resultant mixture was stirred at 60° C. for 24 hours tocause it to undergo a condensation polymerization reaction and therebyobtain a solution of a polyimide precursor represented by the followingstructural formula. Part of the solution was poured into acetone withstirring. As a result, a white solid precipitated. This solid was takenout by filtration, vacuum-dried, and then analyzed for structure by ¹HNMR spectroscopy and IR spectroscopy.

The degree of polymerization n thereof, as determined from integralratio between the signal attributable to main-chain NH and the signalattributable to a terminal olefin, was found to be 10 (number-averagemolecular weight, 3,600).

Using the polyimide precursor solution, a blue light-emitting diodedevice was obtained in the same manner as in Example 1. The polyimideobtained by imidizing the polyimide precursor had the structurerepresented by the following structural formula.

Example 9

A dimethylacetamide solution (3.7 mL) of 1.14 g (10.0 mmol) of1,4-diaminocyclohexane (1,4-DACH) was added dropwise to adimethylacetamide solution (13 mL) of 1.78 g (8.99 mmol) of1,2,3,4-butanetetracarboxylic dianhydride (BDA) and 0.328 g (2.00 mmol)of 5-norbornene-2,3-dicarboxylic anhydride (NBA) at room temperatureover 30 minutes. The resultant mixture was stirred at 60° C. for 24hours to cause it to undergo a condensation polymerization reaction andthereby obtain a solution of a polyimide precursor represented by thefollowing structural formula. Part of the solution was poured intoacetone with stirring. As a result, a white solid precipitated. Thissolid was taken out by filtration, vacuum-dried, and then analyzed forstructure by ¹H NMR spectroscopy and IR spectroscopy.

The degree of polymerization n thereof, as determined from integralratio between the signal attributable to main-chain NH and the signalattributable to a terminal olefin, was found to be 14 (number-averagemolecular weight, 4,800).

Using the polyimide precursor solution, a blue light-emitting diodedevice was obtained in the same manner as in Example 1. The polyimideobtained by imidizing the polyimide precursor had the structurerepresented by the following structural formula.

Comparative Example 1

In a nitrogen atmosphere, 4.22 g (0.0170 mol) ofbis(3-aminophenyl)sulfone was added to 18 mL of N,N-dimethylacetamideand dissolved therein at 25° C. Thereto was added 5.00 g (0.0170 mol) of3,3′,4,4′-biphenyltetracarboxylic dianhydride. The resultant mixture wasreacted with stirring at 25° C. for 4 days to obtain a solution of apolyimide precursor (wholly aromatic polyimide precursor; number-averagemolecular weight, 4,190). Using this solution, a blue light-emittingdiode device was obtained in the same manner as in Example 1.

Comparative Example 2

Forty-five parts by weight of a bisphenol A epoxy resin having an epoxyequivalent of 7,500 (Epikote EP1256, manufactured by Japan Epoxy ResinsCo., Ltd.), 30 parts by weight of an alicyclic epoxy resin having anepoxy equivalent of 260 (EHPE3150, manufactured by Daicel ChemicalIndustries, Ltd.), 22 parts by weight of 4-methylhexahydrophthalicanhydride (hardener; MH-700, manufactured by New Japan Chemical Co.,Ltd.), and 1.2 parts by weight of 2-methylimidazole (hardeningaccelerator; manufactured by Shikoku Chemicals Corp.) were added tomethyl ethyl ketone in such amounts as to result in a concentration of50% by weight. The resultant mixture was stirred at 40° C. for 1 hour toobtain an epoxy resin solution for coating. Using this epoxy resinsolution, a blue light-emitting diode device was obtained in the samemanner as in Example 1.

The solutions (resins) obtained above were examined by the followingevaluation methods. The results obtained are shown in Table 1.

(Light Transmittance)

The resin obtained in each of the Examples and Comparative Examples wasexamined for light transmittance at a wavelength of 400 nm with aspectrophotometer (U-4100, manufactured by Hitachi High-TechnologiesCorp.).

(Heat Resistance)

The resin obtained in each of the Examples and Comparative Examples wasallowed to stand in a 150° C. hot-air drying oven for 100 hours. Theresin which had undergone the 100-hour standing was visually examinedfor transparency. The resins which suffered no change from the originalstate are indicated by “good”, and those which changed from the originalstate are indicated by “poor”.

TABLE 1 Comparative Comparative Example 1 Example 2 Example 3 Example 4Example 5 Example 6 Example 7 Example 8 Example 9 Example 1 Example 2Light 99.7 99.8 99.3 99.6 99.7 99.7 99.9 99.0 99.5 70.1 98.5transmittance (%) Heat good good good good good good good good good poorpoor resistance

It can be seen from the results given in Table 1 that the resins foroptical-semiconductor-element encapsulation according to the inventionhave excellent heat resistance and excellent light-transmittingproperties. Furthermore, the optical semiconductor devices obtained byencapsulating an optical semiconductor element with these resins combineexcellent heat resistance and excellent light-transmitting properties.

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the scope thereof.

This application is based on Japanese patent application No. 2007-292310filed Nov. 9, 2007, the entire contents thereof being herebyincorporated by reference.

Further, all references cited herein are incorporated in theirentireties.

1. A resin for optical-semiconductor-element encapsulation comprising apolyimide which is produced by imidizing a polyimide precursor obtainedby subjecting 5-norbornene-2,3-dicarboxylic anhydride or maleicanhydride, an aliphatic tetracarboxylic dianhydride, and an aliphaticdiamine compound to a condensation polymerization reaction.
 2. The resinfor optical-semiconductor-element encapsulation according to claim 1,wherein the aliphatic tetracarboxylic dianhydride is1,2,3,4-butanetetracarboxylic dianhydride or1,2,3,4-cyclopentanetetracarboxylic dianhydride.
 3. The resin foroptical-semiconductor-element encapsulation according to claim 1,wherein the aliphatic diamine compound is at least one member selectedfrom the group consisting of the compounds represented by the followingformulae (I) to (V):

wherein n is an integer of 4 to 18, and


4. The resin for optical-semiconductor-element encapsulation accordingto claim 1, wherein the polyimide precursor has a number-averagemolecular weight of 3,000 to 50,000.
 5. The resin foroptical-semiconductor-element encapsulation according to claim 1,wherein the polyimide precursor has a degree of polymerization of 5 to1,000.
 6. The resin for optical-semiconductor-element encapsulationaccording to claim 1, which has a light transmittance, as measured at awavelength of 400 nm, of 99% or higher.
 7. An optical semiconductordevice comprising an optical semiconductor element encapsulated with theresin according to claim 1.